Electric resistance heater



Dec. 29, 1964 F. c. wlLLET'r 3,163,841

ELECTRIC RESISTANCE HEATER Filed Jan. 2, 1952 Arme/evs United StatesPatent O 3,163,841 ELECTRIC RESISTANCE HEATER Frank C. Willett, PaintedPost, NY., lassignor to Corning Glam Works, Corning, Nfl/, a corporationof New York Filed Ian. 2',` 1962, Ser. No. 163,494 7 Claims. (Cl.338-292) This invention relates to electrical resistance heaters.

It is an' object of the present invention to provide an electricalresistance heater which is characterized by a large surface to weightratio, and which is largely a ceramic structure yet is rugged and simpleto manufacture.

This and other objects are attained in accordance with this invention inwhich a heater is provided in the form of a` sinteredcer-amic thinwalled honeycomb structure having unobstructed gas passages extendingthrough it andan electrically conducting film on the walls of thehoneycomb.

The structure of the invention is best understood by f reference to theattached drawing in which:

F-IG. l shows a heater in accordance with the invention;

FIG. 2 is an end view of the heater of FIG. 1;

FIG. 3 is-a perspective view of a heating element and its supportingbase;

FIG. 4 is a section along line IV-TV of FIG. 3;

FIG. 5 is a partial sectional View, along line V-V of FIG. 2; and

FIG. 6 is a View similar to FIG. 5 showing but one end of a heater andan additional end element whereby power leads are more easily attachedto the heater.

Referring now to the drawing, a heater 10 is shown enclosed in an outer?member 12' that is made of insulating material such as fiberglass or thelike or is conducting but is insulated from the Working heater portionto protect against electrical shock. Leads 14 and 16, for connection toa power source, are shown extending from the ends of the heater.

The active or working portion of the heater is a sintered ceramichoneycomb structure 18 (see especially FIG. 3) having a plurality ofopenchannels 20 extending through it from its first major end face 22 tothe other end face 24. These channels are separated from one another bythin walls 26 of sintered ceramic material provided in a mannerhereinafter indicated. Walls 26 support electrically conductive films 28(see FIGS. 4 and 5) that extend between faces 22 and 24. Secondelectrically conductive films 30 (FIG. 5) are placed on the edges ofwalls26 at the faces 22 and 24 and are in contact'with the internalfilms 2S at each end of the unit. The leads 14 and 16 are then joined tothe structure as by soldering to film 30.

The sintered ceramic honeycomb structure for this invention suitably isprovided by depositing pulverized ceramic material and a binder on aflexible carrier, corrugating the coated carrier, forming the desiredshape from such coated carriers and thereafter firing the shape tosinter the ceramic particles to produce a unitary structure.

Any known sinterable ceramic material is suitable for making a honeycombfor the present invention. By sinterable ceramic material, I mean aninorganic substance in the crystalline or amorphous state which can becompacted or agglomerated by heating to a temperature near, but below,the temperature at which it melts or has low enough viscosity to deform.Thus glasses, such as borosilicates, soda-lime-silicates,lead-silicates, alumino-silicates, alkaline earth silicates, etc.;refractory compositions such as sillimanite, magnesium silicates,magnesia,

Zircon, zirconia, petalite, spodumene', cordierite, corundum,aluminosilicates, etc.; the glass-ceramics (crystalline materials madefrom glass); or combinations of any of the foregoing materials,.are allsuitable for the present purpose. Refractory compositions consisting ofweight percent of petalite and 5 weight percent of talc, or 75 weightpercent of petalite and 25 weight percent of glass ceramic, areparticularly suitable for low thermal expansion and high thermal shockresistance. A refractory composition comprising corundum (alumina) isparticularly suitable for high-temperature stability. However, theinvention is not dependent upon the sinterable ceramic materialselected' and hence the material which has the most suitable propertiesfor the conditions of its use may be selected.

The particle size of the ceramic material can be of any convenientdimension as dictated by the size of the desired final article and thewall thickness required.

In forming the honeycomb structure, a binder isused to bond the uniredceramic particles to a carrier, to impart green strength to the coatedcarrier and to retain the formed, unred article in the desired shapeprior to firing. Any material having the requisite well-knowncharacteristics ofceramic binders can be used, and these can be oftemporary or permanent character. Typical materials that can be usedinclude such natural materials as gum arabic, colophony and shellac,such synthetic organic resins as acrylate resins, methacrylate resins,alkyd resins, cellulose derivatives, cournarone-indene resins,epoxyresins, furan resins, polyisobutylene, isocyanate resins,fplienolicresins, ypolyarnides, polyesters, resorcinol resins, styrene resins,terpene resins, ureaI resins, vinyl resins, chlorinated parafiins andmelamine resins, and inorganic binders such as sodium silicate andsilicone resins. Preferably, heat curable organic binders that can beremoved by decomposition and volatilization are used. Thecharacteristics of suitable binders are well known to those in theceramic arts and the curing conditions for each generally are publishedin magazines, journals and brochures of the tradey that are universallyavailable.

The carrier provides support for the unfired coating to allow it to beformed to the desiredshape prior to sintering the ceramic coating.kAmong the carriers suitable are inorganic materials including aluminumfoil, aluminum silicate paper, and asbestos cloth and organic materialssuch as cellulose acetate paper, onion skin paper, tea bag paper, nyloncloth, rayon cloth, and polyethylene film. The organic film materialsare preferred as they substantially decompose upon tiring the formedarticle and thus result in a base that is substantially all ceramic.

As noted, the base of the heater is termed a honeycomb structure. Bythis is meant a unitary body having a multitude of open channelsextending from one major surface throughy to the other major surface, ofpredetermined size and shape, each channel being defined by ceramicwalls 26Y `Thus, a multiude of unobstructed gas paths through thestructure is provided. This structure is achieved by assemblingcorrugated coated carriers with or without plain coated carriers. Forexample, corrugated coatedlilms can be assembled with alternate layerslaterally disposed a distance equal to half the width of the individualpattern sothat layers do not nest with each other. Another method offorming ya suitable structure is to lay alternate corrugated anduncorrugated coated carriers on one another until the desired grossdimensions are obtained. Then after tiring and applying the conductivefilms, the honeycomb is enclosed within a ceramic enclosure having thedesired cross section or within a metal or other material suitablyinsulated from the electrical portions in the usual manner. Other waysof producing the base structure, as by using extruded ribbons of aceramic-binder mixture, will be apparent to those skilled in the art.

The ring of the heater base structure is accomplished in the mannernormal in the ceramic arts by placing it in a furnace and heating at arate slow enough to prevent breakage due to thermal shock to atemperature high enough to cause the ceramic particles to sinter. Whilethe heating rates and sintering temperatures are dependent upon both theceramic materials utilized and the shape of the article formed, they arenot critical conditions and suitable conditions are readily determinableby one skilled in the art of firing ceramic articles.

Once the honeycomb substrate or heater structure has been produced, itsinternal surfaces are then provided with an electrically conductive film28 of any desired predetermined electrical resistance. Such films areproduced by conventional processes. Vacuum evaporation of metal ordecomposition of a metal carbonyl are useful in producing a suitable lm.In addition, the film can be formed by chemical deposition of the metalfrom a solution containing a salt of it in which the honeycomb isimmersed. Common materials that are useful for forming the film includecopper, nickel, aluminum, silver and the like. Electroconductive filmscan be provided from metal oxides as set forth in the patent of Davis,Number 2,85 7,294. Obviously, the choice of electrically conductivematerial and its manner of application are Well Within the skill of theart.

After the internal surfaces have been coated, a terminal film 30 isapplied to each of the two major faces 22 and 24 of the structure wherethe air channels terminate. There are many methods by which suchterminals can be applied. For example, low resistance conductive lms canbe produced on the edge surfaces of each channel by spraying, dipping,or painting. Du Pont silver paste No. 4535 is suitable for this purpose.In addition, one can apply a fine Wire mesh or metal screen 34 (see FIG.6),

`for example of copper, over the terminal contact, by

soldering if desired. Thereafter, the leads can'then be attached to thescreen 34 by soldering or any other conventional procedure. Thestructure is then completed by mounting it in the housing designed toprotect the structure but which permits free air movement through thechannels.

The invention will be described further in conjunction with thefollowing example.

A ceramic composition is made consisting of 75 parts by Weight ofpetalite and parts by weight of a glass ceramic having the followingTcomposition in Weight percent: 70 percent SiOg, 18 percent A1203, 5percent TiOz, 3 percent LiOg, 3 percent MgG and 1 percent ZnG. Thecomposition is ball-milled until a particle size of minus 200 mesh(Tyler) is obtained. A solution of the following composition is thenadded to the ball mill for each 2160 grams of ceramic material:

Isopropanol 640 Epoxy resin Hysol 6111 480 Ethyl-acetate a- 860Versarnid 115 180 The term Hysol is the trade name for epoxy resinssupplied by Houghton Laboratories, lnc. The Hysol 6111 is an epoxy resinsolution containing 57 percent by Weight of epoxy resin having aviscosity of about 2.5-4.0 poises at 25 C. The resin has an epoxideequivalent (grams of resin containing 1 g. chemical equivalent of epoxy)of 595i50, and a melting range of 73-85 C. Such resins may be obtainedreadily on the open market as there are a number of producers supplyingsuch resins. Versamid 115 is the tradename of a thermoplastic polymersupplied by General Mills, Inc. It is prepared by condensation ofpolymerized unsaturated fatty acids, such as dilinoleic acid, withaliphatic amines such as ethylene diamine.

The ceramic material and the binder are further ballmilled for aboutthree hours to produce a uniform suspenison. A porous, natural cellulosepaper, commonly known as 31/2 pound tea bag paper, cut to a Width of 4inches is then dipped into the suspension and dried by heating to 120 C.for 2 minutes. The dried, coated paper is then heated to 180 C. andcrimped to produce a pattern, taken in cross-section, in the shape of anisosceles triangle with legs about 0.07 inch long and an open base about0.1 inch wide. Crimped, unfired, coated papers of that type areassembled simultaneously with alternate sheets of coated tea bag paperof the same width that is not crimped; until the desired size isobtained,

The unfired stack is then placed in a furnace and heated at about 350 C.per hour to 700 C. and held at this temperature for about 1 hour. Thestack is then further heated up to 1220 C. at the furnace rate and heldfor 30 minutes. After cooling to room temperature at the furnace rate,it is retired to 1240 C. by heating at a rate of 300 C. per hour andheld for seven hours. The sintered article is then cooled to about 100C. at furnace rate in about 16 hours and removed from the furnace.

The resulting article has about 400 channels per square inch ofcross-section, a total-available-surface of about 80() ft2/fhg, adensity of about 30 pounds per cubic foot, a thermal expansioncoefiicient of minus 2.0 1O7/ C.

Vover the range of 25 to 300 C., and 70 percent free air space measuredin a plane perpendicular to the axis to the stack.

The resulting honeycomb is then provided with an electroconductivecoating on its walls upon heating it to a temperature of about 800 C.and then immersing it momentarily in a 37 percent aqueous HC1 solutioncontaining zinc chloride and antimony chloride in a ratio of 30 parts of-the Zinc chloride to seventy parts of antimony chloride. Then the lowresistance terminal films are applied by painting the ends of thehoneycomb with Du Pont silver paste No. 4535. The electrical leads areattached by soldering them against the silver terminal films after thelatter have been fired to firmly bond them to the honeycomb.

ln the foregoing manner a highly efficient, compact and rugged heater isprovided. The very high surface area per unit of heater makes it evidentthat rapid heating of substantial volumes of air are possible. It is, ofcourse, evident that larger devices for larger loads can be made readilysimply by joining several of the honeycomb bases or by using largercarriers, without loss of anyy of the advantages of the structure.

ln accordance With the provisions of the patent statutes, the principleof the invention has been explained and there has been illustrated anddescribed what is now considered to be its best embodiment. However, itshould be understood that, within the scope of the appended claims, theinvention can be practiced otherwise than as specifically illustratedand described.

What is claimed is:

l. A heater comprising a sintered ceramic thin walled honeycomb shapewith opposed major surfaces and having a plurality of open channelsextending therethrough from one of said surfaces to the other, saidchannels being defined and separated from one another by sinteredceramic walls, an electrically conductive film on the surfaces of saidWalls in each channel extending to the said major surfaces, a lowresistivity electrically conductive lm over the edges of said ceramicwalls at each of said major surfaces, said low resistivity films beingin electrical contact sassari a plurality of open channels extendingthrough said shape from one or said surfaces to the other, anelectrically conductive hlm on the surfaces of said Walls in eachchannel extending to the said major surfaces, a low resistivityelectrically conductive lin over the edges of said ceramic walls at cachof said major surfaces, said low resistivity films being in electricalcontact at each surface With said films in said channels, and anelectrical lead from each of said loW resistivity films.

4. A heater in accordance With claim 3 including a conductive mesh oneach of said low resistivity films, said electrical leads being joinedto said mesh.

5. A heater comprising a ceramic thin Walled honeycomb shape havingopposed major surfaces, said shape being formed by sintering a honeycombassembly con1- prising a crimped ceramic coated carrier, therebydelining a plurality of open channels extending through said shape fromone or said surfaces to the other, an electrically conductive lm on theWalls in each channel extending to the said major surfaces, a lowresistivity electrically conductive lm over the edges of said ceramicWalls at each of said major surfaces, said low resistivity films beingin electrical Contact at cach surface with said films in the channels,and an electrical lead from each of said low resistivity iilms.

6. An electric heater comprising a ceramic honeycomb structure providedwith a multitude of open-end channels extending therethrough, electricalresistance films coating the Walls of said channels, and electricalconducting means joined to said Iilms for connecting them with anelectric power source.

7. An electric heater comprising a ceramic honeycomb structure providedwith a multitude of open-end channels extending therethrough, electricalresistance films coating the Walls of said channels, a pair ofelectrical leads adapted to be connected to an electric power source,and low resistivity electrically conductive iilms on said honeycombstructure electrically connecting said resistance lilins to said leads.

References Qited in the tile of this patent UNITED STATES PATENTS

1. A HEATER COMPRISING A SINTERED CERAMIC THIN WALLED HONEYCOMB SHAPEWITH OPPOSED MAJOR SURFACES AND HAVING A PLURALITY OF OPEN CHANNELSEXTENDING THERETHROUGH FROM ONE OF SAID SURFACES TO THE OTHER, SAIDCHANNELS BEING DEFINED AND SEPARATED FROM ONE ANOTHER BY SINTEREDCERAMIC WALLS, AN ELECTRICALLY CONDUCTIVE FILM ON THE SURFACES OF SAIDWALLS IN EACH CHANNEL EXTENDING TO THE SAID MAJOR SURFACES, A LOWRESISTIVITY ELECTRICALLY CONDUCTIVE FILM OVER THE EDGES OF SAID CERAMICWALLS AT EACH OF SAID MAJOR SURFACES, SAID LOW RESISTIVITY FILMS BEINGIN ELECTRICAL CONTACT AT EACH SURFACE WITH SAID FILMS IN SAID CHANNELS,AND AN ELECTRICAL LEAD FROM EACH OF SAID LOW RESISTIVITY FILMS.